Sendmail is a Mail Transfer Agent, which is the
program that moves mail from one machine to
another. Sendmail implements a general
internetwork mail routing facility, featuring
aliasing and forwarding, automatic routing to
network gateways, and flexible configuration.

Postfix is an attempt to provide an alternative to the widely-used Sendmail program. Postfix attempts to be fast, easy to administer, and hopefully secure, while at the same time being sendmail compatible enough to not upset your users.

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. In style it is similar to Smail 3, but its facilities are much more extensive. Exim is in use at many sites around the world.

The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, Webmail, and mailing list services within a single consistent framework. Individual components can be enabled or disabled at will. Courier implements basic Web-based calendaring and scheduling services integrated in the Webmail module.

getmail is intended as a simple, secure, and
reliable replacement for fetchmail. It retrieves
email (either all messages, or only unread
messages) from one or more POP3, SPDS, or IMAP4
servers (with or without SSL) for one or more
email accounts, and reliably delivers into
qmail-style Maildirs, mboxrd files, or through
external MDAs (command deliveries) specified on a
per-account basis. getmail also has excellent
support for domain (multidrop) mailboxes,
including delivering messages to different users
or destinations based on the envelope recipient
address.

Kolab is a groupware system that provides a
comprehensive and flexible solution for
workgroups. Email, contacts, and group calendars
can be managed via the Web, Windows/Outlook, and
Linux/KDE. It supports offline synchronization and
full seamless support of mixed client environments
because the Kolab-XML storage format is fully open
and avoids MAPI/TNEF.

Citadel is an advanced messaging and collaboration system for groupware and BBS applications. Users can connect to Citadel using any telnet, WWW, or client software. Among the features supported are public and private message bases (rooms), electronic mail, real-time chat, paging, shared calendaring, address books, mailing lists, and more. Unlike other collaboration servers, Citadel provides its own data stores and is therefore extremely easy to install; you don't have to "bring your own" email and database because they're built in. The server is multithreaded and scalable. In addition, SMTP, IMAP, and POP3 servers are built-in for easy connection to Internet mail. Citadel is both robust and mature; it has been in production since 1987.

NullLogic Groupware is a modular Web-based groupware and email system designed for contact management and event scheduling. Additional features include private messaging, public discussion forums, webmail, shared bookmarks, file sharing, call tracking, order processing, an XML-RPC API, and integrated POP3 and SMTP servers.